This invention is an apparatus for making electrolytically deposited metal foil. It is particularly adapted to the production of copper foil by electrodeposition for use in making printed circuits and to the production of a copper-containing electrolyte for use in electrodeposition.
Stemming from the pioneering work of Thomas A. Edison and others around the turn of the twentieth century, as evidenced by Edison U.S. Pat. No. 1,417,464, issued in May 1922, continuous electrodeposition of metal foils, particularly of copper, iron, and nickel, has been carried out successfully in many commercial operations. Another early example of such work is given in U.S. Pat. No. 2,044,415, issued to Yates in June 1936. Copper, by far the most deposited metal foil on a world-wide basis, is one of three major raw materials necessary for the manufacture of printed circuit boards, the other two required raw materials being a base or board of fiberglass or phenolic material and an epoxy or other bonding chemical. Following lamination to a suitable electrically insulating base and etching to remove unwanted copper, the foil becomes the medium for the physical support and electrical interconnection of electronic components in a large number of devices.
Commercial copper-foil operations have been developed in the United States of America and Canada, Europe, and Asia by a number of manufacturers. However, each of these manufacturers has tended to develop pieces of foilmaking equipment as custom items on a proprietary, in-house basis. There is no source for the basic equipment used to manufacture foils in the standard widths and thicknesses. These standard foil thicknesses are 2 ounces per square foot (70 microns thick); 1 ounce per square foot (35 microns thick); 1/2 ounce per square foot (17 microns thick) and 1/3 ounce per square foot (12 microns thick). The standard foils are and probably will continue to be in high demand. Apart from standardization that has been achieved on specific widths and gauges required in manufacturing, production of the foil has usually relied upon internally developed equipment without overall standardization, although there have been and continue to be attempts at standardization of the basic equipment within particular organizations. Foil manufacturers have relied on their own varied designs and equipment assembled from a variety of components procured from a variety of manufacturers. Inevitably, that approach has been costly, resulting in many failures and much unnecessary replication of design effort on what by other standards is rather simple equipment.
Such a situation has significantly slowed the growth of the foil business, which today remains both somewhat slow to change techniques and also very capital-intensive. Moreover, emerging electronics industries in several parts of the world, particularly in Asia, have been discouraged or prohibited from acquiring captive foil operations because they have no manufacturer of standard equipment to turn to. This is not the case in the follow-on or allied parts of the printed-circuit industry, where there are many competitive manufacturers of specialized items of equipment. For example, where are over the world several alternative manufacturers of laminating presses and of machinery to print and etch the laminates to produce circuit boards. In developing nations an emerging electronics industry typically integrates backwards. Assembly of electronic equipment comes first, followed by the acquisition of printed circuit manufacturing capabilities, followed by laminating capabilities. A major road block to full backwards integration to produce all raw materials is the lack of standardization and the unavailability of foil-making machinery.
Most of the copper foil that is used in printed circuits is electrodeposited on metal drums that range in diameter from about 15 inches (38 cm.) to about 118 inches (3 meters) and in width from about 40 inches (90 cm.) to about 118 inches (3 meters). Spacings between anodes and drums are generally in the range of the order of 1/4 inch (6 mm.) to 1 inch (25 mm.). Electrolytes are typically aqueous solutions of sulfuric acid and copper sulfate, with concentrations of copper as metal ranging from about 50 to about 115 grams per liter and of sulfuric acid ranging from about 70 to about 140 grams per liter. The electrolyte is supplied at solution flow rates of about 25 to 1250 gallons per minute (100 to 5000 liters per minute) and at temperatures ranging from about 50 C. to 70 C. Current densities range from about 150 amperes per square foot to about 1500 amperes per square foot (1500 to 15,000 amperes per square meter). Properties of electrodeposited copper foil may also be influenced by the presence of additives in the electrolyte, including chloride ions from the addition of hydrochloric acid and organic materials such as glue, gelatine, hydroxyethyl cellulose, and the like.
Some of the problems in producing electrodeposited foil and some alternative solutions to the problems are discussed in a number of patents including U.S. Pat. No. 4,869,971, issued Sep. 26, 1989, entitled "Multilayer Pulsed-Current Electrodeposition Process." This patent summarizes the state of the art in electrodeposition and also shows how to electrodeposit multiple layers from a single solution. U.S. Pat. No. 4,898,647, issued Feb. 6, 1990, entitled "Process and Apparatus for Electro-plating Copper Foil," teaches a version of a rotating drum for electrodeposition and also a concentric anode. An alternative version of an anode for electroplating copper foil is given in U.S. Pat. No. 4,913,973, issued Apr. 3, 1990, entitled "Platinum-Containing Multilayer Anode Coating for Low pH, High Current Density Electrochemical Process Anodes." This patent describes some of the problems that are overcome in that patent by using a specially treated plating of platinum as the anode surface.
A process of treating foil after it is electroplated is given in U.S. Pat. No. 4,952,285, issued Aug. 28, 1990, entitled "Anti-Tarnish Treatment of Metal Foil." The metal foil in question is copper or copper alloy which is treated with an aqueous solution of chromic acid and sulfuric acid, after which it is rinsed and dried. Another example of technology that is currently in use is taught by U.S. Pat. No. 4,956,053, issued Sep. 11, 1990, entitled "Apparatus and Process for the Production of Micro-Pore Free High Ductility Metal Foil." The '053 patent teaches the use of an electroplating drum with an anode of lead, alloys of lead and antimony, antimony, or alloys of lead, antimony, and silver. U.S. Pat. No. 4,961,828, issued Oct. 9, 1990, entitled "Treatment of Metal Foil," teaches a system for producing metal foil electrolytically and treating it chemically.
U.S. Pat. No. 4,976,826, issued Dec. 11, 1990, entitled "Method of Making Electrodeposited Copper Foil," teaches adding a water-soluble cellulose ether to the electrolyte to assist in depositing a smooth foil. Another drum for electroplating metal foil is taught by U.S. Pat. No. 5,019,221, issued May 28, 1991, entitled "Electroplating Drum with High Current-Carrying Capability."
All of these patents show aspects of the present state of the art in the making and treating of electrodeposited foil, particularly copper foil. They also share one or more of the following disadvantages. Those that show electrical connections to the shaft of a rotating drum show the connection on only one end of the shaft, which causes some difficulties in maintaining a uniform current density at the surface of the drum. Those that show a supply of electrolyte entering along the bottom of a drum do not show any apparatus for maintaining turbulent flow of the electrolyte during electrolysis to supply metal ions in a relatively high concentration at the surface of the drum and to remove the oxygen evolved in the reaction from the reaction site. None of these references shows an apparatus for dissolving copper in sulfuric acid efficiently.